The exploration of gas and oil involves drilling a borehole into the ground through the geological formations of interest. Drilling mud is used to lubricate and cool the drill bit, to assist in bringing the drill cuttings to the surface, to provide sufficient hydrostatic pressure within the well to prevent the bore collapsing under the forces experienced underground, and to prevent the influx of fluid from the formation while drilling. The drilling mud can be either aqueous based or oil or synthetic based.
When the drilling of the borehole is completed a tubular known as a casing is secured within the borehole with cement. This insures permanence of the borehole and it prevents entry of formation fluid, other than from the formation which is being produced. It is common procedure to pump the cement down the inside of the casing to the bottom and then up the outside of the casing, between the casing and the borehole wall, until it fills and lines the section of the annulus to be cemented. That is, the space between the casing and the borehole wall. It is important that the cement lining be free of voids, that is, sections between the casing and borehole wall that is void of cement. It is also important that the cement be well bonded to both the casing and borehole wall. If good bonding is not established between the cement and the casing and borehole wall surfaces, then production from one reservoir could enter another and be lost; an undesirable fluid from one reservoir could enter the production zone of another; or the pumped fluid of an injection well could end up in the wrong zone. To ensure good bonding between cement and the appropriate surfaces, it is necessary to remove substantially all traces of drilling mud on the casing and the borehole wall. Incomplete mud removal can leave a channel behind which could prevent total isolation of a production zone. Unfortunately, the substantial complete removal of mud often proves extremely difficult. Various methods and mechanisms have been devised in an attempt to achieve complete mud removal and to provide a complete cement lining, free of voids, between the casing and the borehole wall.
One type of method includes the use of spacer fluids ahead of the cement. That is, liquids that will literally wash the mud off of the casing and borehole wall. These fluids can be pumped so that they are positioned between the cement and the mud. Such fluids can be of the type known as "chemical washes" which are usually low-viscosity liquids containing surfactants and mud thinners, or diluents. They can also be those known as "spacers", which are rather more viscous, gel-like liquids that are primarily used to form a buffer between the cement and the mud. Although it is not always clear in the literature whether a particular fluid is a spacer or a chemical wash, a spacer is generally characterized as a thickened composition that functions primarily as a fluid piston in displacing the mud. Frequently, spacers contain an appreciable amount of weighting materials, as well as fluid loss control agents. Chemical washes, on the other hand, are generally thin, or relatively non-viscous, fluids which are effective principally as a result of turbulence, dilution, and surfactant action on the mud and mud filter cake. Chemical washes may contain some solids to act as an abrasive, but the solids content is generally significantly lower than in spacers because chemical washes are typically too low in viscosity to have good solids carrying capacity. For purposes of the present invention, the term "spacer" or "spacer fluid" means both chemical washes as well as the more conventional meaning for the more viscous spacer fluids.
With respect to displacing the drilling fluid system with a water base system, operators also find it beneficial to clean the cased hole or inner diameter of the casing and fill the cased hole section with a water base mud, water base drill in fluid, seawater, brine, brine completion fluid, or completion fluid. In wells drilled with oil or synthetic base muds, the casing is run and cemented. Normally, the operator will then run cased hole logs and do testing of the casing and liner tope and may even tieback the casing to surface. After all this has been done and the cased hole, which is full of oil or synthetic base drilling fluid, is ready to be displaced to the next stage, we would then utilize a series of spacers in the non-aqueous design to displace the oil or synthetic base mud from the well bore, clean the inside of casing(s) and all tubulars, leaving all surfaces water wet and then install a water base fluid.
In completion operations, the hydrocarbon bearing reservoir is prepared for production. In the case where the well bore contains a casing string, the casing and cement is perforated, as those of ordinary skill in the art will appreciate. It has been found that fines and debris within the completion fluid tend to enter the subterranean reservoir and damage and/or destroy the permeability of the hydrocarbon zone after completing the well thereby decreasing the production. When an operator displaces a drilling mud with a water based fluid system, many of the problems previously listed with reference to cementing are faced with completion fluids. The displacement and cleanup of oil or synthetic base drilling fluids is necessary to protect the integrity of the water base fluid so it does not get contaminated with oil, to remove solids, so during completion these solids do not enter the formation and cause damage to the producing formation or cause any problems to downhole equipment or tools such as packers and to allow corrosion inhibitors, added to completion fluid to protect the casing from corrosion, to work properly by offering them a clean and water wet surface to coat. Therefore, an adequate cleaning of the down hole tubulars and removal of the drilling fluid system from the well bore is required.
Aqueous based chemical washes are taught in U.S. Pat. No. 4,207,194. These chemical washes are of the type containing one or more of: (a) at least one surfactant to remove water based drilling muds from a borehole; and (b) at least one surfactant to enhance the bonding of cement to the wall of a borehole and the casing. There is also included an effective amount of a fluid loss additive such as a mixture of at least two oil soluble particulate resins, one of which remains hard and friable, and the other of which is soft and pliable (at the temperature to be encountered in the well) when dispersed in an aqueous medium.
U.S. Pat. No. 4,108,779 teaches a water-in-oil emulsion spacer fluid which is compatible with drilling fluids and cement compositions and which is comprised of a hydrocarbon oil, salt water, an emulsifier which is comprised of a first and second fatty acid, and a surfactant-dispersant.
U.S. Pat. No. 5,101,902 teaches an aqueous spacer composition compatible with drilling muds and cement slurries which spacer composition is comprised of a biopolymer selected from the group consisting of scleroglucan and biopolymers produced by the bacteria Azotobacher indicus as an anti-settling means, a fluid loss controlling means, a wetting agent selected from the group consisting of at least one polyethoxyl ester of a C.sub.9 -C.sub.12 fatty acid, and a weighting means.
U.S. Pat. No. 4,588,032 teaches a fluid spacer composition for use in well cementing, which composition is comprised of an aqueous fluid, an admixture of nonylphenols ethoxylated with from 1 to 14 moles of ethylene oxide, and preferably, at least one member selected from the group consisting of a sulfonated linear C.sub.8 to C.sub.18 straight chain alcohol ethoxylated with from 2 to 20 moles of ethylene oxide, a low molecular weight aliphatic glycol ether containing from about 3 to about 12 carbon atoms per molecule and an alcohol containing from 3 to 8 carbon atoms ethoxylated with 2 to 4 moles of ethylene oxide together with any conventionally utilized spacer additives, if desired, such as viscosifiers, weighting agents, and the like.
While there are various chemical washes and spacer fluids in commercial practice today, there is still a need in the art for ever improved techniques for displacing drilling fluids and leaving the borehole surfaces clean. Further, there is a need for an improved technique that will remove the drilling fluid system and provide for a clean environment for installation of a water based fluid. The water based fluid may be used as a completion fluid.